1. Field of the Invention
The present invention relates to mechanisms for extracting water from a web of material, and more particularly, from a fibrous web being processed into a paper product on a papermaking machine. Specifically, the present invention is an impermeable belt designed for use on a long nip press on a papermaking machine, and having grooves on its outer surface for the temporary storage of water pressed from the fibrous web, and a method for constructing the impermeable belt.
2. Description of the Prior Art
During the papermaking process, a fibrous web is formed on a forming wire by depositing a fibrous slurry thereon. A large amount of water is drained from the slurry during this process, after which the newly formed web proceeds to a press section. The press section includes a series of press nips, in which the fibrous web is subjected to compressive forces designed to remove water therefrom. The web finally proceeds to a drying section which includes heated dryer drums around which the web is directed. The heated dryer drums reduce the water content of the web to a desirable level through evaporation.
Rising energy costs have made it increasingly desirable to remove as much water as possible from the web prior to its entering the dryer section. The dryer drums are often heated from within by steam and related costs can be substantial, especially when a large amount of water needs to be removed from the web.
Traditionally, press sections have included a series of nips formed by pairs of adjacent cylindrical press rollers. In recent years, the use of long press nips has been found to be advantageous over the use of nips formed by pairs of adjacent rollers. Long press nips are so called because they have a greater extent in the longitudinal, or machine, direction than those formed by pairs of adjacent rollers. The longer the web can be subjected to pressure in the nip, the more water can be removed there, and, consequently, the less will remain to be removed through evaporation in the dryer section. This is accomplished in a long press nip by virtue of the fact that any given portion of the fibrous web takes a longer time to pass through the nip.
The present invention relates to long nip presses of the shoe type. In this variety of long nip press, the nip is formed between a cylindrical press roller and an arcuate pressure shoe. The latter has a cylindrically concave surface having a radius of curvature close to that of the cylindrical press roller. When roller and shoe are brought into close physical proximity, a nip is formed which can be five to ten times longer in the machine direction than one formed between two press rollers. This increases the so-called dwell time of the fibrous web in the long nip while maintaining the same level of pressure per square inch pressing force used in a tworoller press. The result of this new long nip technology has been a dramatic increase in dewatering of the fibrous web in the long nip when compared to conventional nips on paper machines.
A long nip press of the shoe type requires a special belt, such as that shown in U.S. Pat. No. 4,946,731 to Dutt. This belt is designed to protect the press fabric supporting, carrying, and dewatering the fibrous web from the accelerated wear that would result from direct, sliding contact over the stationary pressure shoe. Such a belt must be made with a smooth impervious surface that rides, or slides over the stationary shoe on a lubricating film of oil. The belt moves through the nip at roughly the same speed as the press fabric, thereby subjecting the pres fabric to minimal amounts of rubbing against stationary components of the press.
A typical configuration for a long nip pres involves the use of two press fabrics. The components in the nip can be listed in the following sequence: a grooved cylindrical press roller, a first press fabric, the fibrous web or nascent paper sheet, a second press fabric, the special belt, the lubricating film, and the arcuate pressure shoe.
Many of the long nip presses currently in commercial operation are of the above configuration. However, this so-called double-felted long nip press is limited to certain types of paper grades. To expand the application of the long nip press to finer paper grades and newsprint, which, during manufacture, come into contact with a smooth press roll, it will be necessary to develop a single-felted long nip press.
In a single-felted long nip press, the components in the nip would be listed in the following sequence: a smooth cylindrical press roller, the fibrous web or nascent paper sheet, the press fabric, the special belt, the lubricating film, and the arcuate pressure shoe. As implied by this sequence, the fibrous web actually contacts the cylindrical press roller in a single-felted long nip press.
In the double-felted long nip press, the water pressed from the fibrous web can pass into either of the two press fabrics, as well as into the grooves provided in the cylindrical press roller. In the single-felted long nip press, two of these sinks for water storage have been eliminated. There is only one felt and, because the fibrous web contacts the cylindrical press roller directly, this roller must have a smooth surface. Clearly, this loss of temporary water storage space must be made up in another fashion, because it is absolutely mandatory that voids be provided in the nip of the press, so that water pressed from the paper sheet in a single-felted long nip press will have a place to go.
Two approaches have been taken to achieve a satisfactory solution of this problem. Canadian Patent No. 1,190,779 shows a long nip press belt having voids on the felt side. These voids arise from the weave pattern of the base fabric and the absence of impregnation on the felt side of the belt, and provide a place into which liquid can be transferred during passage of the fibrous web, press fabric, and special belt through the nip.
The other approach is represented by the previously noted U.S. Pat. No. 4,946,731. This shows a long nip press belt having grooves on the side facing the felt, or press fabric, to store water pressed from the fibrous web.
To make a belt of the variety shown in U.S. Pat. No. 4,946,731, a base fabric having the form of an endless loop is coated, both inside and outside, with a polymeric resin, such as polyurethane. This coating is applied in such a manner as to completely impregnate the structure of the base fabric rendering it impervious to liquids. It should further be smooth and of uniform thickness.
On the outer surface of the coated belt are cut a plurality of grooves, which may, for example, lie in the machine direction or cross-machine direction. The grooves define channels for water pressed from the fibrous web in the long press nip, and are separated from one another by what may be referred to as land areas. These areas are portions of the coating not cut away to create grooves.
Alternatively, the grooves may be provided by means of an embossing roll run over the outer surface of the coated belt before the polymeric resin has had sufficient time to cure to a hardened state.
Clearly, the manufacture of a grooved, long nip press belt of this variety requires a complicated series of steps. The present invention has been motivated by a desire to streamline the production of a grooved belt by rendering unnecessary the steps of coating both sides of the base fabric, and of cutting, by one of several possible means, grooves into the outer coated surface thereof.